Radio frequency transmission circuit



May 23, 1933. w. VAN B. ROBERTS RADIO FREQUENCY TRANSMISSION CIRCUIT Filed June 12, 1931 Patented May 23, 19333 entree stars-s rarest orric llj WALTER VAN RADIO FREQUENCY TRAlTSlVIISSION CIRCUIT Application filed June 12,

My present invention relates to radio fre: quency transmission circuits, and more par ticularly to radio signaling systems employing a novel and improved type of radio fre quency amplicati-on equalization means.

The equalization of amplification at different radio frequencies has been accomplished in the past by the use of combined electromagnetic and electro-static coupling between a source of signal energy, as for example the anode circuit of an electron discharge tube, and an oscillation circuit, the latter being, in the case of a tube, the circuit into which the tube is workin Capacity coupling alone gives an amplification which increases with a decrease of frequency. Magnetic coupling by itself permits amplification which increases with increasing frequencies. The combination of the two couplings in proper proportions gives an amplification which may be equal at two difierent frequencies, and which is, as a matter of practical experience, fairly uniform over a range of frequencies.

New, I have discovered a novel method of, and devised means for, obtaining equal amplification at two different frequencies, which method and means operate upon an entirely difierent principle from methods heretofore used in the prior art. Briefly, advantage is taken of certain peculiar characteristics of a radio frequency transmission line having uniformly distributed series inductance and shunt capacity. It has been found that when a source of radio frequency voltage of a certain internal resistance is connected to a resonant system composed of lumped inductance and lumped capacity, having a certain input resistance at resonance, if the connection is made through a transmission line, having distributed series inductance and shunt capacity, designed accord-" ing to a predetermined relationship, then equal amplification can be secured at two different radio frequencies.

It may, therefore, be stated that it is a prime object of my presentinvention to provide a source of radio frequency voltage of a predetermined internal resistance, a'r'esonant system composed of lumped inductance 331. Serial No. 543,82.

B. ROBE 'rs, or rnmonron, new JERSEY, .assrsnon To name coitronn'rron or AMERICA, A oonronA-rron or DELAWARE 1 4 and capacity, which system has a predeter- K mined input resistance "at resonance, and a transmission line of predetermlned design provided with distributed series inductance and shunt capacity, the design of a line being determined by the wave length of the energy to be transmitted. v a Another important object of the present invention is to provide a method of radio frequency energy reception wherein amplifica tion of collected energy at different frequencies is equalized by means of a-transmission path of uniformly distributed inductance and capacity.- 1 Another object of the invention is to equalize the amplification of a stage of tuned radio frequency amplificatiomin a radio receiver, at two predetermined frequencies, and thus inakethe amplification over a range of frequencies more uniform.

Still other objects-of the present invention are to improve generally the efiiciency of amplification of radio frequency energy at different radio frequencies, and to particu larly provide a radio frequency transmission path between resonant circuits in a radio re- 'ceiver.

The novel features which I believe to be characteristic of my invention are, set forth in particularity in the appended claims, the

invention itself, however, as to both its organization and method'of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one arrangement whereby my inductor 1 including the uniformly wound inductance L. The uniformly distributed shunt capacities are represented by dotted lines, each capacity being designated by the reference letter C. A finite number of shunt condensers are shown to represent uniform distribution.

The parallel conductors may be as long as desired, the dotted linesextending from each terminal of each conductor denoting the fact that the conductors may be extended in length. Those skilled in the art know that a coil need not'actually be used in the conductor 1, but that two parallel conductors inherently possess uniformly distributed series inductance and shunt capacity as shown in Fig. l. The input terminals of the transmission line may be considered as coupled to a generator of voltage 6, the internal resistance of the generator being represented by the resistor R and the load at the other end of the line being represented by the resistor R V The action of a transmission line, in the case of the transmission of radio frequency energy, is well known in the art, and consequently it will only'be necessary in the pres ent description to note those characteristics of the line which are utilized inthe present invention. These characteristics may be stated as follows: (1) The characteristic impedance of a transmission line as shown in Fig. 1, or as in Fig. 2, may be defined by the expression where Z and c are the numbers of henries inductance in series in the transmission line per centimeter of its length, and the number of farads capacity across the line per centimeter, respectively.

(2) One wave length of the line at frequency f is equal to (3) 7 If it is desired to deliver the maximum amount of power. into the load R shown in Fig. 1, through an arbitrary length of transmission line, which transmission line has an arbitrary value of characteristic impedance, then it is necessary to connect the output end of the line to the load B, through a transformer which will match the characteristic impedance to the load resistance; it being, additionally, necessary to connect the input terminals of the line to the generator 6 through a transformer which will match the characteristic impedance of the line to the internal resistance R, of the generator.

(at) If, however, the design of the trans mission line can be made to suit the object (for example, maintaining maximum power input to the load resistance R then the transformer at the ends of the line may be dispensed with, while still obtaining maximum power transfer; this result is secured provided the length of the line is an odd multiple of a quarter wave length, the characteristic impedance of the line being made the geometric mean between the generator resistance R and the load'resistance R (5) If the length of transmission line is an exact number of half wave lengths, then the input impedance of the transmission line, when its output is connected to the load R will be R regardless of the characteristicimpedance of the line.

Referring now to Fig. 2 wherein there is shown a radio receiver circuit embodying the principles of the present invention, the circuit comprises the usual grounded antenna system A, G wherein a radio frequency transformer M is utilized for transferring collated radio frequency signal energy to the tunable input circuit of an electron discharge tube 1. lVhile the tubeis shown as being a triode, it is to be clearly understood that it may also be of'the screen grid type. The input circuit of the tube comprises the secondary coil 3 of the transformer M, the coil being in shunt relation with a variable tuning condenser 4.

The anode of the tube 1 is shown connected to the cathode of the "tube 1 through the conductor '1 ofthe transmission line shown in Fig. 1, the primary winding L, the conductor 2 .of the transmission line shown in Fig. 1, and a source of potential 13.. In order to maintain stable operation of the amplifier, a neutralizing condenser 5 is connected between the anode of the tube 1 and the lower side of the variable condenser L, N egative bias for the grid of tube '1 is supplied from a source 6 having its positive terminal connected to the cathode, and its negative terminal connected to the midpoint of the coil 3 through a resistor 7. The latter is utilized to suppress parasitics, and possesses a magnitude of about 300 ohms;

The second stage of radio frequency amplification comprises an electron discharge tube 2', preferably of the same character as the tube 1', theinput of the tube including the coil L the latter being coupled to the coil L A variable tuning condenser 4 is con nected in shunt relationwith the coil L, the condensers 4 and 4 being adapted for simultaneous and similar variation by any well known means, the latter being conventionally represented by the dotted lines 8. A source of current 6 is utilizedfor applying a negative bias to the grid of the tube 2', while the source B is employed for supplying positive potential to the anode of the tube.

i It is tobe understood that the invention is not limited to the use of the battery sources shown, but that the circuits of the receiver shown in Fig. 2 may be energized from a com mon source ofdirect current, or rectified pulsating current. The output circuit of the second stage of radio frequency amplification will be coupled to any number of succeeding stages of any desired type. For example, additional stages of radio frequency amplification may be'employed, whether tuned or untuned, or the second stage may be followed by a detector, and one, or more, stages of audio frequency amplification. It is, also, within the scope of the present application to utilize the twostages of radio frequency amplification shown herein in conjunction with the frequency changer device of a super heterodyne receiver. It will be seen that even if the mutualinductance between coils L and L is not enough to result in a large amplification when the transmission line 1, 2 is not inserted between the tube 1 and the coil L the amplification can nevertheless be made substantially a maximum even while utilizing a small value of mutual inductance between the aforementioned coils. This may be accomplished by inserting a quarter wave length of transmission. line between the output of tube 1' and the coil L which line has a characteristic impedance equal to the square root of the product of the anode circuit resistanceof the tube 1 and the input resistance measured between the terminals a, b of coil L when the secondary circuit, which comprises coil L and condenser 4-, is tuned to resonance.

In order to illustrate the application of the principles of the present invention to a particular case, it being understood that the eX- ample given is merely for the purpose of illustration and is not to be construed as a limitation, assume that it is desired to produce equal amplification at 600 kilocycles, and at 1200 kilocycles. Suppose that byempirical methods the number of turns of coil L is adjusted until the amplification at 1200 kilocycles, without the transmission line, is the same as the greatest amplii cation that can be obtained at 600 kilocycles, by increasing the number of turns of coil L Now, insert a transmission line between the output terminals d, e of the tube 1 and the terminals a, b of the coil L whose length is a quarter wave length at 600 rilocycles, and whose character istic impedance is the geometric mean between the tube resistance and the transformer input resistance at resonance measured at 600 kilocycles, the primary being designed as stated above. 7

As a result of the insertion of the transmission line it will be found that at 600 kilocycles, the power transferred, and hence the amplification, is the greatest that can be attained with that particular tube and tuned circuit. At 1200 kilocycles on the other hand, the

' transmission line plays no part. 'This has been pointed out in item (5) referred to above. This is necessarily so, since the transmission line is a half wave long at 1200 kilocycles, and, therefore, the amplification is simply the sameas if the transmission line were absent, and this is according to the construction described, the amount of amplification, that is the best possible at 600 kilocycles.

In the foregoing example it has been assumed that the transmissionline 1, 2 is substantially free of losses. If it is attempted to make a transmission line extremely compact (for example so short physically as to go conveniently into a radio receiver) the losses in the line become considerable, and a considerable departure is naturally required from the design given as an example. In the case of a transmission line with considerable loss it will probably be found easier to determine the entire design by empirical methods. I

In operating a line between a tube of 10,000 ohms internal resistance, and a transformer whose impedance at the low frequency end of the broadcastrange is approximately a thou sand ohms, it is obvious that the transmission line should have a characteristic impedance of a little over 3000 ohms. From the formula it is seen that the impedance may be'decreased by increasing-the value of distributed capacity, and one way of doing his would be to put the return side of the line 2 closer to, or inside of, a continuous coil. L forming the series inductance. c r

i It is, of course, obvious that-amplification may be equalized at other frequencies than the two given in the illustrative example, and, also, that the amplification may be given predetermined values at two different frequencies as well as equal values. It is, also, to be understood that the transmission line 1, 2

need not be employed between the output circuit of a tube and a succeeding tunable resonant circuit, but may be utilized between an antenna circuit and the tuned input circuit of an electron discharge tube. 'Furthermore, while in Fig. 2 the transmission line has been represented as a pair of parallel conductors, one of which is wound as a'coil, it is entirely feasible in the present invention to employ two parallel conductors, and rely upon the inherent uniformly desired series inductance and shunt capacity of the conductors.

' While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited tothe particular organization shown and described, but that manv modifications may be 7 made without departing from the scope of my invention as set forth in the appended claims.

l/Vhat I claim is:

1. In combination, a source of radio frequency voltage of a predetermined internal resistance, a resonant circuit tunable over a desired frequency range and consisting of lumped inductance andlumped capacity coupled to said source, the resonant system having a predetermined input resistance at resonance, and a non-tunable transmission line having distributed series inductance and shunt capacity for coupling said source and resonant circuit, said line being an odd number of quarter wave lengths long at the lower limiting frequency of said range. a

2. In combination, a source of radio frequency current having a predetern iined value of internal impedance, a resonant'circuit tunable over a range of signal frequencies and having a pair of input terminals, a nontunable transmission line for connecting said source and resonant circuit,-the line having effectively uniformly distributed series inductance and shunt capacity and being an odd number of quarter wave' lengths long at a frequency in the vicinity of the lower limit of said range, the characteristic ime pedance of said line at said frequency being the geometric mean between the impedance of said source and the impedance measured at said frequency between the pair of input terminals having a fixed voltage ratio to said resonant circuit.

8. In combination with a stage of tuned radio frequency amplification including 'a space discharge tube and a succeeding tuned circuit adapted to be tuned over a desired signal frequency range, means for coupling the circuit and the output of said tube, said means comprising a. non-tunable transmission line having uniformly dist *ibuted series inductance and shunt capacity, said line being an odd number of quarter wave lengths long at a frequency in the region of the lower limit of said range.

4. In combination, a source of radio fre-- quency voltage of a predetermined internal resistance, a resonant circuit tunable over a desired frequency range and consisting of lumped inductance and lumped capacity coupled to said source, the resonant system having a predeterminedinput resistance at resonance, and a non-tunable transmission line having distributed series inductance and shunt capacity for coupling said source and resonant circuit, said line being so designed that it matches the impedances of the source and circuit at the lower limit of said desired frequency range, while acting as adirect connection at the upper limiting frequency of said range.

5. In combination, a source of radio frequency voltage of a predetermined internal resistance, a resonant circuit tunable over a desired frequency range and consisting of lumped inductance and lumped capacity coupled to said source, the resonant system having a predetermined input resistance at resonance, and a transmission line having distributed series inductance and shunt capacity for coupling saidsource and resonant circuit, said line being an odd number of quarter Wave lengths long at the lower limit of said range and being designed in such a manner as to act as an impedance matching element at long waves, and as a direct connection at short waves.

6. In combination with an amplifier circuit comprising a source of signal energy and an amplifier having a resonant input circuit adapted to be tuned over a range of frequencies, an arrangement for producing in said amplifier predetermined values of amplification at one frequency and at a frequency twice as great, said arrangement includinga transmission line connecting said source to said tunable input circuit, a primary coil connecting the output of saidtransmission line and said tunable input circuit, the mu:

tual inductance. between said primary coil and tunable circuit being so chosen as to produce saidpredetermined amplification at said higher frequency when said primary is directly connected to the output of the tube, said line being an odd number of quarter wave lengths long at said lower frequency and having a characteristic impedance so chosen as to produce said predetermined amplification at said lower frequency.

WALTER VAN B. ROBERTS.

Lin! 

